Method for detecting and correcting an error in a multilevel memory and memory device implementing the method

Abstract: The invention relates to a multi-level storage device comprising: at least a first plurality (CP) of cells (C-0...C-N), each adapted for storing an identical first number (greater than one) of binary data (B0...B7), and at least a corresponding second plurality (WP) of cells adapted for storing a second number of error check and correcting words (W-0...W-7) equal to said first number, said words (W-0...W-7) being respectively associated with sets of binary data, each comprising at least one binary data for each cell in said first plurality (CP). In this way, many of the known error correction algorithms can be applied to obtain comparable results to those provided by binary memories. In addition, where multi-level cells are used for storing the error check and correcting words, the device dimension requirements can also be comparable.

Description: This invention relates to a method for checking and correcting errors in a multi-level memory as set forth in the preamble of Claim 1, as well as to a memory device implementing the method. The problem of error correction in memories of conventional design has already been addressed heretofore. The US Patent 4,958,352 for example, discloses a semiconductor memory provided with an ECC (Error Check and Correcting) circuit. The memory comprises a first cells matrix for the principal information, and a second cells matrix for the ECC information. Associated with each information word, an 8-bit word in the example, is an error check and correcting word, a 4-bit word in the example, which only allows a single error to be located and corrected (this being generally found adequate). Each binary data is stored into a different one of the memory cells. The US Patent 4,335,459 similarly discloses a conventional system of performing error correction on a memory made of binary memory cells. Understandably, by providing memory cells which can store, for example, four binary data, it becomes possible to store, in the instance of the above example, each information word into two cells of the first matrix and the corresponding error check and correcting word into a cell of the second matrix. Using Hamming codes, as the codes which provide single-error check and correcting words of shortest length, the relation 2 M =N+1+M applies, where N is the bit length of the information word, and M is the bit length of the ECC word. There are, of course, other kind of codes which allow a larger number of errors to be checked and/or corrected, but they require longer error check and correcting words. The subject is exhaustively dealt with in Jiri Adamek, "Foundations of Coding", John Wiley & Sons, 1991. Multi-level memories are currently the subject of intensive research work because of the growing demand for semiconductor storage devices of large capacity, in particular non-volatile ones. In memories of this kind, write and/or read errors are much more likely to occur, and steps must be taken to prevent or at least reduce them. In investigating the operation of multi-level memories, in particular of the EEPROM and Flash EPROM types, it has been found that when a cell is programmed improperly, more than one binary data appear in that cell which are incorrect. Of course, it would be possible to use a code which allows a large number of errors to be checked and corrected, but this results in the error corrective matrix attaining impractical dimensions, well in excess of those of the informational matrix. It is an object of this invention to provide a method which enables errors in a multi-level storage device to be checked and corrected, fits the error modelling for such memories, and affords good error allowance, without expanding the dimensions of the device where the latter is integrated to a chip; another object being the provision of a storage device architecture which can implement the above method. These objects are achieved by the method defined in Claim 1 and the storage device having the features defined in Claim 8. Further advantageous aspects of the invention are set forth in the subclaims. In a multi-level storage device comprising at least one plurality of cells, each adapted to store an identical first number of binary data bits, which first number is greater than one, if for each plurality, at least a second number of error check and correcting words is provided which is equal to said first number, and if said words are respectively associated with sets of binary data bits, each comprised of one binary data bit for each cell in the plurality, then many of the known error correction algorithms can be applied directly to obtain comparable results to those provided by binary memories. Furthermore, if multi-level cells are used to store the error check and correcting words, their effect on size is also comparable. The invention will be further explained in the following description, to be read in conjunction with the accompanying drawings, in which: Figure 1 shows a block diagram for a storage device according to this invention; and Figure 2 shows a plurality of multi-level cells and a corresponding number of ECC words, in accordance with the invention. The method of this invention will now be discussed with reference to Figure 2. It provides error checking and correcting capabilities in a multi-level storage device of the type which comprises at least one plurality CP of cells C-O...C-N, each adapted to store an identical first number (eight in Figure 2) of binary data bits B-0...B-7. This method provides, for each plurality, at least a second number (eight in Figure 2) of error check and correcting words W-0...W-7. This second number is equal to said first number, and the words W-0...W-7 are respectively associated with sets of binary data, each comprising one binary data of each cell in the plurality CP. It will be appreciated that there exist several possibilities for selection of the binary information contained in the sets. Lacking any special reasons for doing otherwise, such sets would be discrete sets. Of course, the sets might include several binary data of one or more cells. The simplest choice would be that of including in the set "0" the binary data B0 of the cells C-0...C-N in the plurality CP; in the set "1", the binary data B1...; and in the set "7", the binary data B7. The error check and correcting words W-0...W-7 are computed from the associated set by means of an algorithm which permits of error checking and correcting in said set. As previously mentioned, there are several codes which can do this. Provided that the number N of the cells in a plurality has been suitably selected, the probability of a single error is a reasonable one; in this case the most suitable code (the one requiring the shortest ECC word) is the Hamming code: when N is selected equal to 120, the length of the words W-0...W-7 is 7. The error check and correcting words can be stored into another plurality WP of cells of a suitable type to store said first number (eight) of binary data; in this case, any additional occupation of the chip area due to the error correction is drastically reduced, in Figure 2 by a factor of eight. The error check and correcting words W-0...W-7 include a third number (seven in Figure 2) of binary data B0...B6; the other plurality WP of cells may consist of a fourth number (seven in Figure 2) of cells equal to said third number. In that case, it is convenient if each binary data of each word is stored into a different cell of the other plurality. The straightest choice consists, as shown in Figure 2, of storing into a first cell of the other plurality WP all binary data B0 of the words W-0...W-7; into a second cell, the binary data B1...; and into a seventh and last cell, the binary data B7. Understandably, there exist other viable options. The words W-0...W-7 could also be stored respectively into eight different multi-level memory cells. In the instance of Figure 2, such cells would be somewhat underutilized in that information would be stored therein which corresponds to seven binary data, whilst their capacity is of eight binary data; the eigth binary data could be advantageously used for storing the parity of the corresponding error check and correcting word. In Figure 2, the cells in the two pluralities CP and WP are depicted as if they belonged to a word line WL of a memory matrix; each cell C-0...C-N is associated with a bit line BL-0...BL-N; and the words W-0...W-7 are associated with an error bus EB formed, in this example, of 7 lines equal to the length of the words and the number of cells provided for storing them. With reference now to Figure 1, where the storage device comprises at least a first matrix IA of cells arranged in rows and columns, for each row (as in Figure 1) and/or column, at least said second number of error check and correcting words are provided, and each row (as in Figure 1) and/or column forms a different plurality. These error check and correcting words may then be stored into a second matrix EA of cells having the same number of rows (as in Figure 1) or columns as the first matrix IA. A storage device according to this invention will now be described with reference to Figure 1. This device is basically comprised of: a) at least a first plurality CP of cells, each adapted for storing an identical first number of binary data, which number is greater than one, and b) at least a corresponding second plurality WP of cells adapted for storing a second number of error check and correcting words equal to the first number, such words being respectively associated with sets of binary data, each comprised of a binary data of each cell in the first plurality CP. The same considerations made in connection with the method description apply here to the sets. The device of Figure 1 comprises plural (four) storage sections divided into information storage sections DS0, DS1, DS2, DS3, and error storage sections ES0, ES1, ES2, ES3. Each informational section consists of 32 first pluralities CP of cells and each error section consists of 32 corresponding second pluralities WP of cells, and further comprises: a) a line decoder LD being input a line address LA and outputting 32 line signals L-0...L-31; b) an information section decoder DSD being input 4 groups of 120 bit lines from the information sections DS0, DS1, DS2, DS3 and a section address SA, and outputting one of the four groups of bit lines; c) an error section decoder ESD being input 4 groups of 7 bit lines from the error sections ES0, ES1, ES2, ES3 and a section address SA, and outputting one of the four groups of bit lines; d) ECC means connected to the outputs of the decoders DSD and ESD, operative to check and correct any errors appearing in the contents of the cells in a first addressed plurality by the use of the contents of cells in a second addressed plurality, and to generate corrected information; and e) a column decoder RD being input said corrected information and a column address RA, and being adapted to select, based on this address, some of the corrected information for supply on the device output U. The cells of the information sections DS0, DS1, DS2, DS3 jointly form an information matrix IA, while the cells of the error sections ES0, ES1, ES2, ES3 form an error matrix EA. Advantageously, all the cells of the matrix IA and the matrix EA are adapted for storing the same number of binary data, eight in the example of Figure 2. In the circuit of Figure 1, each information section DS represents a sub-matrix of the matrix IA; each row constitutes a different first plurality CP; each error section ES represents a sub-matrix of the matrix EA; each row constitutes a different second plurality WP; for each row in the sub-matrix DS, that is for each first plurality CP, there is a corresponding second plurality WP, this biunivocal relationship being exemplified in the Figure for two particular pluralities by a double-headed arcuate arrow joining two blocks of the matrix IA and EA. Understandably, in order to read and write a storage device of this type, special methods are required which would depend on its construction. The method of this invention for reading corrected information in such a multi-level storage device basically comprises the steps of: a) sending a read address to said device consisting, in Figure 1, of a line address LA, a section address SA, and a column address RA; b) by means of the line address LA and the section address SA, selecting and reading a first plurality CP of cells; c) by means of the line address LA and the section address SA, selecting and reading a second plurality WP of cells; d) checking and correcting any errors appearing in the contents of the cells in the first plurality CP by using the contents of the cells in the second plurality WP; e) generating corrected information; and f) by means of the column address RA, selecting some of the corrected information for supply on the device output U. The method of this invention for writing correctable information from information to be stored into one such multi-level storage device, basically comprising the steps of: a) sending the information to be stored to said device; b) sending, to said device, a write address consisting, in Figure 1, of a line address LA, a section address SA, and a column address RA; c) storing such information into a first plurality CP of cells identified by the write address, specifically in Figure 1, by the line address LA in combination with the section address SA, at a location specified by the write address, specifically in Figure 1, by the column address RA; d) computing error check and correcting words from the contents of the cells in the identified plurality CP; and e) storing said computed words into a second plurality WP of cells identified by the write address, specifically in Figure 1, by the line address LA in combination with the section address SA. To revert to the storage device, it should be understood that there are several architectures to implement a traditional storage device, and consequently, there will be many different options for positioning, dimensioning, arranging and addressing the plurality of error check and correcting cells, all of which allow the method according to this invention to be implemented, and each technician will be able to pick up the most suitable option for the basic storage architecture.

Claims: A method for checking and correcting errors in a multi-level storage device of the type which comprises at least one plurality (CP) of cells each adapted for storing an identical first number of binary data bits (B0...B7), said first number being greater than one, characterized in that, for each plurality (CP), at least a second number of error check and correcting words (W-0...W-7) is provided which is equal to said first number, and that said words (W-0...W-7) are respectively associated with sets of binary data bits, each comprising one binary data bit of each cell in said plurality. A method according to Claim 1, wherein said sets may comprise several binary data of one or more cells. A method according to Claim 1, wherein said error check and correcting words are computed from the associated set using an algorithm which allows for checking and correcting a single error in said set. A method according to Claim 1, wherein said error check and correcting words are stored into another cell plurality (WP) of the type adapted for storing said first number of binary data. A method according to Claim 4, wherein said error check and correcting words (W-0...W-7) are composed of a third number of binary data (B0...B6), wherein said other plurality (WP) is composed of a fourth number of cells equal to said third number, and wherein each binary data of each word is stored into a different cell in said other plurality. A method according to Claim 1, wherein said storage device comprises at least a first matrix (DS1) of cells arranged in rows and columns; wherein for each row and/or column, at least said second number of error check and correcting words is provided; and wherein each row and/or column constitutes a different plurality (CP). A method according to Claim 6, wherein said error check and correcting words are stored into a second matrix (ES1) of cells having the same number of rows and columns as said first matrix (DS1). A multi-level storage device, comprising: a) at least a first plurality (CP) of cells, each adapted for storing an identical first number of binary data bits, said number being greater than one, and characterized by further comprising: b) at least a corresponding second plurality (WP) of cells adapted for storing a second number of error check and correcting words equal to said first number, said words being respectively associated with sets of binary data bits, each comprised of a binary data bit of each cell in said first plurality (CP) . A device according to Claim 8, comprising plural storage sections (DS0...DS3;ES0...ES3) consisting of at least said first plurality and said corresponding second plurality of cells, and further comprising: a) first decoder means (LD,DSD) operative to select and read the first plurality (CP) of an addressed section in said plurality of sections; b) second decoder means (LD,ESD) operative to select and read the second plurality (WP) of said addressed section; c) error check and correcting means (ECC) connected to the outputs of said first (DSD) and second (ESD) decoder means, operative to check and correct any errors appearing in the contents of the cells in the addressed first plurality (CP) by the use of the contents of the cells in the addressed second plurality (WP), and to generate corrected information; and d) third decoder means (RD) operative to select, based on an address, some of said corrected information and to supply it on the device output (U). A device according to Claim 8, wherein the cells in said second plurality (WP) are adapted for storing an identical first number of binary data, said first number being greater than one. A device according to Claim 8, comprising at least a first matrix (DS1) of cells arranged in rows and columns, wherein each row and/or column constitutes a different first plurality (CP), and wherein a corresponding second plurality (WP) is provided for each row and/or column. A device according to Claim 11, further comprising a second matrix (ES1) of cells having the same number of rows and columns as said first matrix (DS1), and adapted for storing said error check and correcting words.

Inventors: CAPPELLETTI PAOLO, CAPPELLETTI, PAOLO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: G06F11
IPC codes: G11C29, G06F11, G11C7, G11C11, G06F12